1. Field of the Invention
This invention relates to electronic packages for integrated circuits, including methods and apparatus related to manufacturing, and in particular, electronic chip carrier packages with solder bump electrical connections between chip substrate pads and the chip carrier package.
2. Background of the Invention
Current manufacturing techniques utilize two primary methods for coupling integrated circuit chip substrates to chip carrier packages. The first is wire bonding, whereby each of the I/O pad terminals on a chip substrate is sequentially wired to corresponding pads on a chip carrier package. The second method is flip chip attachment (FCA) in which all the I/O pads on a chip are first terminated with a solder material. The chip is then flipped over and the solder bumps are aligned and reflowed in a reflow furnace to facilitate all of the I/O connections with bonding pads on the chip carrier package. An advantage achieved by the flip chip process is its suitability to high I/O density and greater reliability of the interconnections—formed as compared to wire bonding processes.
There are a variety of methods presently used to form solder bumps on a chip substrate. Often, the formation of solder bumps is carried out by a method of evaporating lead and tin mixtures through a mask for producing the desired configuration of solder bumps on the chip substrate. Techniques of electrodeposition of such mixtures have also been used to produce solder balls in flip chip packaging.
Another popular technique is the process of solder paste screening. However, the continued evolution of integrated circuit manufacturing processes toward progressively higher density circuits necessitates a correspondingly higher I/O pad density and tighter pitch constraints for pad terminals. For current processes, it is not unusual for a design to contain more than 1000 I/O pads. As a result, the solder paste screening technique becomes less practical to implement. Moreover, since the solder paste is normally applied directly to the substrates through a screen mask which contains holes aligned to the paste receiving pads on the substrate, any problems occurring during the process may also result in substantial rework of the substrate, thereby increasing the probability of damage to the substrate and significantly impacting manufacturing throughput.
A more recently developed injection molded solder technique dispenses molten solder instead of solder paste. An advantage of this process results from very little volume change occurring between the molten solder and the resulting solder bump. This process is typically practiced by first filling with solder a mold containing holes or cavities aligned to the pads on the substrate. Next, the filled mold is placed in close proximity to a substrate which contains solder receiving pads and onto which flux material has typically been dispensed in a thin layer over the substrate. When the solder in the mold is heated to a melting temperature in a furnace, surface tension reduction causes the solder to ball up and intimately contact the solder receiving pad, which is normally covered with gold or other solder wetting alloy. A wiper may be used after the molten solder fills the holes to remove excess solder. However, when this technique is used on large substrates, the balling up of the solder may be insufficient to ensure intimate contact between the solder in the mold cavities and solder receiving pads on the substrate and thus the solder balls may not adequately adhere to the substrate contact pads.
One prior art technique for overcoming the difficulties of known processes in forming solder bumps for integrated circuit to package interconnections is described in U.S. Pat. No. 6,003,757 entitled “Apparatus for Transferring Solder Bumps and Method of Using,” issued Dec. 21, 1999 and commonly assigned to International Business Machines Corporation. This patent describes a method and apparatus to maintain a solder mold in intimate contact with the solder receiving substrate, for example a semiconductor wafer, during a solder reflow operation such that the solder in the mold is transferred to solder wettable pads on the receiving substrate. A uniform pressure on the mold substrate assembly is necessary to ensure that all solder segments from the mold cavities are able to contact all solder wettable pads on the substrate at the time that the solder becomes molten. As described in U.S. Pat. No. 6,003,757, the apparatus applies such a uniform pressure until physical disassembly by human intervention as when opening the lid of the clamshell assembly of the apparatus releases the pressure. Such human intervention must occur after the mold-substrate assembly has exited the reflow furnace and cooled. Due to the pressure, the molten solder has maintained the shape of the mold cavity in which it was located and in effect, is somewhat adhering to most or all of the mold cavity surfaces. Although this is not a metallurgical bond in the sense of the solder-substrate pad interface, which is a strong metallurgical bond, separating the mold from the cavity nonetheless requires a certain tensile force and care must be taken to avoid any shearing motion. Both of these latter conditions risk unintended separation of the metallurgical bond between the solder and substrate pad. To reduce such risk, the mold-substrate assembly is subjected to a second solder reflow operation after the uniform pressure has been physically released. At this stage without any compressive forces present and with gravitational forces minimized by orienting the assembly such that the lighter substrate is on top, the remelted solder, now metallurgically bonded to the substrate pad, will tend to partially ball up, thus forcing the mold and the substrate to partially separate and facilitate physical separation of the two. Once successfully separated, it is often desired to have perfectly rounded solder bumps on the substrate in order to optimize subsequent assembly operations, suggesting yet another solder reflow operation of the substrate alone.